This invention relates to a method for detecting breakdowns in an electrostatic filter which is fed from an a-c voltage source via a rectifier, a high voltage transformer and a final control element, and wherein the overstepping of a given difference voltage value of single measured values of equal phase position of successive half-waves of the filter voltage is used as breakdown criterion.
A method of this general type is known, for example, from U.S. Pat. No. 4,138,232.
It is customary to detect a breakdown through the increase of the primary current. The sole detection of the current peaks beyond the nominal current is, however, not sufficient, since, during operation, disruptive discharges whose peaks do not exceed the nominal current frequently occur. These low current disruptive discharges, too, must be detected by the control. The current detection must therefore be designed so that current peaks after overstepping of the operating current just then prevailing by a certain percent are recognized and that thereafter control processes are triggered.
In addition to this indirect detection of disruptive discharges provided on the primary side of the voltage installation, there is direct detection to be used on the high voltage side. In the latter, one utilizes, e.g., the voltage collapse at the moment of breakdown. It makes no difference whether or not the disruptive discharge current has exceeded the nominal current limit. Since the voltage collapse also occurs at low current disruptive discharges, it is detected within the present half-wave. By indirect detection through the primary current, on the other hand, detection takes place as a rule only through the charging current peak of the half-wave following the discharge. At the moment of disruptive discharge, in fact, the primary current shows only a slight reaction. Hence, direct detection of the breakdown on the high voltage side is to be regarded as the most favorable.
A method for high voltage side detection of disruptive discharges may consist, for example, in continuously comparing the voltage amplitudes of successive half-waves of the voltage fluctuations at the separator, and using a given deviation of correlated measured values as the criterion for a breakdown.
Since experience has shown that flashovers occur after the voltage maximum of the half wave, it may, according to the solution of the above-named U.S. Pat. No. 4,138,232, suffice in many cases to compare the voltage waveforms after the amplitude maximum. To this end, for example, at fixed moments, individual voltage values in the descending flank of the separator voltage may be picked up and stored, these voltage values being compared with the measured voltage values of a following half wave at the corresponding moments, displaced by the duration of the period. If the comparison shows a considerable deviation, this may serve as criterion for the flashover.
Such an observation of breakdowns has proved successful; however, the setting of a sufficient difference over the entire filter voltage range is somewhat of a problem, for one thing because at low voltages this value would have to be made smaller than at relatively high voltages.